北部北上帯の亜帯区分と渡島帯・南部秩父帯との対比：

Abstract

The North Kitakami Belt of Northeast Japan, which consists of a Jurassic accretionary complex, is divided by the Iwaizumi Tectonic Line into two sub-belts, the Kuzumaki-Kamaishi and Akka-Tanohata sub-belts, which differ in terms of the dominant feldspar type in sandstone and the presence or absence of Paleozoic oceanic sedimentary rocks. We carried out a detailed study of the geology of the boundary area between these two sub-belts in the west Akka area in Iwaizumi Town, Iwate Prefecture.Our survey resulted in the identification of three tectonostratigraphic units. From lower to upper (east to west), these are the Takayashiki (mixed facies), Seki (coherent facies of chert-clastic sequences associated with thick sandstone layers), and Otori (coherent facies of chert-clastic sequences and dominant mudstone) units. Paleozoic fossils occur only in the Seki and Otori units, supporting the criterion proposed previously for identifying the location of the Iwaizumi Tectonic Line. However, the dominant feldspar composition of sandstone changes gradually from plagioclase-dominant in the Otori Unit, through equal amounts of both plagioclase and Kfeldspar in the Seki Unit, to K-feldspar-dominant in the Takayashiki Unit. Thus, the feldspar compositions from west to east are not consistent with the proposed location of the tectonic line. The tectonostratigraphic units and sandstone compositions of the North Kitakami Belt are well correlated with those of both the Oshima and South Chichibu belts, which are the northern and southern tectonic extensions of the North Kitakami Belt, respectively. However, the oldest age of each unit is different between these three geologic belts. Paleozoic fossils have not been reported from the Shimonosawa Unit of the Oshima Belt, which corresponds to the Seki Unit, whereas the Sambosan Unit of the South Chichibu Belt, which can be correlated with the Akka-Tanohata sub-belt, yields Paleozoic fossils.

Recognition of an Early Triassic accretionary complex in the Nedamo Belt of the Kitakami Massif, Northeast Japan: New evidence for correlation with Southwest Japan

“…High-P/T metamorphic rocks in both Southwest Japan and the Kitakami Massif might have formed in the same arc-trench system of the circum-Pacific orogenic belt (e.g., Tsujimori & Itaya, 1999). Based on lithological similarities, such as Megalodon-fossil-bearing Upper Triassic Suzuki et al (2007), Uchino and Suzuki (2020), and other studies referred to in these four papers limestone (Sano et al, 2009) and "Torinosu-type" Upper Jurassic limestone (e.g., Murata, 1962;Onuki, 1959), part of the Jurassic accretionary complexes in the Akka-Tanohata Subbelt of the North Kitakami Belt have been considered the Sanbosan Unit of the southern belt in the Chichibu Belt (e.g., Isozaki & Maruyama, 1991;Takahashi et al, 2016) (Figure 11). However, the Early Carboniferous Tsunatori Unit in the Nedamo Belt cannot be correlated with any geotectonic units in Southwest Japan.…”

Sedimentology and biostratigraphy of the upper Triassic carbonates from the Shiriya cape, North Kitakami Belt, Japan

“…The Sea of Japan to the West and the Pacific Ocean to the East define the other natural borders of the North Kitakami Belt. The latter is divided into two sub‐belts (i.e., the Akka‐Tanohata and the Kuzumaki‐Kamaishi sub‐belts) whose limits are defined by the Iwaizumi Tectonic Line (the Seki‐Odaira Fault), extending northwest‐southeast (Takahashi et al, 2016). These two sub‐belts are dominated by trench‐fill deposits of middle Jurassic (Suzuki, Ehiro, et al, 2007; Suzuki, Yamakita, et al, 2007; Yoshihara et al, 2002) and late Jurassic (Matsuoka & Oji, 1990; Nakae & Kamada, 2003) ages, and characterized by a younger age eastward.…”

Identification of conodont fossils in pelagic deep‐sea siliceous sedimentary rocks using laboratory‐based X‐ray computed microtomography

“…The geological setting of the Okoshizawa locality is detailed in Ehiro et al (2008). The pelagic deepsea sedimentary rocks in Okoshizawa comprise siliceous claystone and bedded chert of the Otori Formation (Sugimoto 1974), the lower portion of a coherent chert-clastic sequence (the Otori Unit of Takahashi et al 2016) within the Jurassic accretionary complex of the North Kitakami Belt (Ehiro et al 2008;Takahashi et al 2016). The strata in Okoshizawa have yielded late Carboniferous to early Permian conodonts and corresponds to the lowermost part of the Otori Unit (Ehiro et al 2008).…”